This invention relates to optical fibre filters.
There are many applications for optical fibre band-pass and band-stop filters in optical fibre systems, allowing spectral filtering of optical signals.
An example of the use of such devices is the filtering of amplified spontaneous emission after an erbium-doped fibre amplifier. Current commercially available devices, e.g. devices based on thin film technology, fibre Fabry-Perot interferometers and, recently, fibre gratings in combination with circulators, are in one way or another based on some bulk optic devices, and therefore have high insertion losses and are expensive.
Several all-fibre based filters have been proposed, one based on a mismatched twin-core (TC) fibre designed to phase-match at the filtering wavelength [see publication references 1,2,3]. Grating assisted coupling in a mismatched twin-core fibre can also be used to implement a band-pass filter [4,5]. However, these techniques require a bespoke fibre for each possible centre wavelength, and therefore a large stock of fibre must be built up. There has not been a technique which offers the required high quality performance and flexibility and an easy implementation of filters at any desired wavelength.
Recently, due to the surge of interest in wavelength-division-multiplexing systems, spectral dependent loss with designed profiles has become a very interesting topic for many who want to achieve a wide bandwidth Er-doped optical fibre amplifier by gain shaping. The dominant technology for achieving this so far has been long period photosensitive gratings written in fibres using a UV laser to couple a guided mode into a cladding mode [6]. This method allows accurate control of the filter response and therefore can implement the complicated spectral loss profiles required. However, the response of these gratings is highly sensitive to any index change of the core. This can be caused by a change of temperature, strain or decay of the photosensitive index change and out-diffusion of hydrogen, should low temperature hydrogenation be used. This makes it very difficult to predict the final device response during fabrication and, worse still, other stabilisation technologies have to be employed to maintain the same grating response at different operational conditions, i.e. a change of temperatures or strain. To reduce the temperature sensitivity, specially designed fibres have to be used [7,8].
So, there is a need for a fibre-based filter having reproducible optical characteristics.
This invention provides a method of fabricating a band-processing optical fibre filter having a centre wavelength xcex0xe2x80x2, the method comprising the steps of:
(i) radius-reducing a mismatched multi-core optical fibre having a core phase matching wavelength (before radius reduction) of xcex0 and a radius (before radius reduction) of a0, to a reduced radius Ra0, where R=xcex0xe2x80x2/xcex0; and
(ii) providing light input and output connections to a section of the radius-reduced multi-core fibre, so that input light is launched into one of the cores of the multi-core fibre section and output light emerges from one of the cores of the multi-core fibre section.
The invention provides an elegantly simple band-processing (e.g. band-pass or band-stop) filter fabrication method and a corresponding filter.
The invention recognises that the coupling between cores of a multi-core optical fibre is (a) highly wavelength dependent, and (b) highly dependent on the core radius in the multi-core fibre. So, if a band-pass or band-stop filter is produced by coupling light from an input core of a multi-core fibre to another core and outputting light from the input or another core, the wavelength range at which this takes place can be controlled by adjusting the radius of the cores of the multi-core fibre.
The invention further recognises that an elegant and convenient way of doing this is to reduce the radius of the multi-core fibre, e.g. by a thermal process. This provides a reduction in the overall fibre radius which can be measured or predicted easily, but more importantly provides a corresponding reduction in the radius of each core of the multi-core fibre. So, by achieving a desired degree of radius reduction of the fibre as a whole, the desired core radius can easily be achieved.
This is quite different to the techniques described, for example, in publication reference [2] where the fibre initially has cores so far separated that substantially no coupling occurs. A radius reduction from 140 xcexcm to 38 xcexcm is required in that reference to move the cut-off wavelength from 1 xcexcm to 980 nm.
The skilled man will appreciate that a mismatched multi-core fibre is one in which the effective radius and the effective refractive index are both different between cores. The skilled man will also appreciate that the term xe2x80x9cfilterxe2x80x9d does not exclude devices with a net gain at at least some wavelengths. The term xe2x80x9cfilterxe2x80x9d simply implies a device having a wavelength-dependent response. Similarly, the term xe2x80x9ccentre wavelengthxe2x80x9d does not necessarily imply a symmetrical wavelength-dependent response. It is simply a term used widely in the art to refer to a wavelength substantially at the (positive or negative) peak of the device""s response.
Embodiments of the invention can provide a highly reproducible and accurate way of tuning the coupling wavelength of a mismatched TC fibre, allowing the coupling wavelength to be accurately positioned at any wavelength over a few hundred nanometre wavelength range. The diameter of the TC fibre used is reduced on a coupler rig. The highly controllable reduction of TC fibre diameter is used to adjust the coupling wavelength of the TC fibre.
In prototype embodiments, a tuning range of 550 nm has been demonstrated, only to be limited by the measurement set-up used, and not by the technique itself.
Using embodiments of the invention, band-pass and band-stop filters of very high spectral quality have been implemented as prototypes to demonstrate the potential of the technique. The accurate tuning technique, in combination with filter strength (or extinction ratio) tuning by adjusting the length of the TC fibre, enables spectral dependent loss of various profiles to be implemented by having several of the filters in series, demonstrating the accuracy and flexibility of the technique.
The filter response is intrinsically very stable in terms of temperature change and strain change, allowing easy packaging. This is because the length of the filter is an odd multiple (e.g. 1) of the coupling length between the coresxe2x80x94generally of the order of a centimetre or more. Fractional changes in length due to temperature or strain variations simply move the device slightly away from the coupling length (this does not change the coupling wavelength significantly but can affect the filter strength slightly). A main benefit is that temperature and strain have very little effect on the core parameters. In contrast, in grating devices they would change the grating pitch and in interferometric devices they would change the resonance of the device, both of which are very much more sensitive to tiny length changes. So, devices made according to the present invention can be much less sensitive to environmental conditions than previous interferometric or grating-based filters.
The invention also provides a band-processing optical fibre filter having a centre wavelength xcex0xe2x80x2, the filter comprising:
a section of radius-reduced mismatched multi-core optical fibre having a core phase matching wavelength (before radius reduction) of xcex0 and a radius (before radius reduction) of a0, the section being radius-reduced to a reduced radius Ra0, where R =xcex0xe2x80x2/xcex0, and
(ii) light input and output connections to the multi-core fibre section arranged so that input light is launched into one of the cores of the multi-core fibre section and output light emerges from one of the cores of the multi-core fibre section.